AGC198606: A gas-bearing dark matter minihalo?

Abstract

We present neutral hydrogen (H i) imaging observations with the Westerbork Synthesis Radio Telescope of AGC198606, an H i cloud discovered in the ALFALFA 21cm survey. This object is of particular note as it is located 16 km s −1 and 1.• 2 from the gas-bearing ultrafaint dwarf galaxy Leo T while having a similar H i linewidth and approximately twice the flux density. The H i imaging observations reveal a smooth, undisturbed H i morphology with a full extent of 23 × 16 at the 5 × 10 18 atoms cm −2 level. The velocity field of AGC198606 shows ordered motion with a gradient of ∼25 km s −1 across ∼20 . The global velocity dispersion is 9.3 km s −1 with no evidence for a narrow spectral component. No optical counterpart to AGC198606 is detected. The distance to AGC198606 is unknown, and we consider several different scenarios: physical association with Leo T, a minihalo at a distance of ∼150 kpc based on the models of Faerman et al. (2013, ApJ, 777, 119), and a cloud in the Galactic halo. At a distance of 420 kpc, AGC198606 would have an H i mass of 6.2 × 10 5 M , an H i radius of 1.4 kpc, and a dynamical mass within the H i extent of 1.5 × 10 8 M .

Hubble Space Telescope Imaging of the Ultra-compact High Velocity Cloud AGC 226067: A Stripped Remnant in the Virgo Cluster

“…The main stellar body of AGC 226067 consists primarily of blue stars in several distinct clumps. A second clump of blue stars ∼1.6 arcmin to the northwest of the main body of AGC 226067 is seen, as has been noted in previous work (Adams et al 2015;Sand et al 2015;Beccari et al 2016Beccari et al , 2017a. In contrast, an uncataloged Virgo cluster dwarf galaxy serendipitously in our ACS field of view (which we dub Dw J122147+132853) consists almost entirely of RGB stars, which we discuss below.…”

“…We have constructed a CMD of AGC229490 corresponding to the position of the gas cloud, and its rough dimensions (R.A.: 12:21:53.8, decl. : +13:29:08, with size 45″×45″; Adams et al 2015). The CMD is largely consistent with several equal area background CMDs, possibly with an overdensity of ∼5 or fewer stars.…”

“…7 M e in a deconvolved area of 3.08×10 7 pc 2 (Adams et al 2015), giving a gas surface density of 0.49…”

“…The coordinates and radial size for AGC 226067 and DwJ122147 +132853 are approximate,but theycorrespond to the CMD region used in Figure 2. a From Adams et al (2015).…”

“…Follow-up highresolution H I observations with the Very Large Array showed that AGC 226067 broke up into two distinct H I clouds (Adams et al 2015)-the primary H I source is coincident with the optical/UV counterpart, while the secondary H I component also has an optical/UV counterpart offset by 0 5. Data from the Multi Unit Spectroscopic Explorer (MUSE) at the VLT revealed 38 separate H II regions scattered throughout the main and secondary stellar body of AGC 226067, with a nearly uniform metallicity of 12 log O H 8.37 á + ñ = ( ) (for the main body), more metal-rich than expected if AGC 226067 was a gas-rich dwarf galaxy (Beccari et al 2017a).…”